Because it is a great way of forgetting something from the pass and look forward to the future. For me, it is a great way to think and even you writing your own music helps by expressing your feelings as a song but it also depends on how bad the tragedy is but I still think that music really helps people to get over a tragedy.
Music bringing memories flooding back. Memories of ones before tragedy took them flood the mind. Those same memories help the survivors stay connected to the lost ones. Music in all it diversity can bring comfort at different levels. The warm feelings of care, love and companionship that result from memories triggered by music are some the of best in times of solitude after a tragedy.
Music can help one get over a tragedy. Some types of music can have a clam and soothing affect on a person suffering from a tragedy. Music therapy has become very common for psychologists treating certain types of patients. Music therapy can be a better way to treat someone, as opposed to just prescribing medication.
While the effects of a tragedy are overwhelming and more often than not life-changing and music sounds like a rather small and insignificant thing compared to one, I honestly believe it to be one of the elements that aids a person in picking himself up and moving on. Certain types of music can have a very uplifting effect on people.
I do not believe that music has any inherent properties which would allow someone to get over a tragedy. Anecdotally, listening to music can aid recall of a certain time or place and, in doing so, could lead someone to dwell on a past tragedy as easily as reliving past glory.